Electrostatic developer composition and method therefor



United States Patent ELECTROSTATIC DEVELOPER COMPOSITION AND METHODTHEREFOR Chester F. Carlson, Pittsford, N. assignor, by memoassignments, to Haloid Xerox Inc., Rochester, N.Y., a corporation of NewYork No Drawing. Filed Nov. 29, 1957, Ser. No. 699,437

. 12 Claims. (Cl. 252-621) This invention relates in general toxerography and in particular to a developer composition of improvedperformance in the art of xerography.

In xerography it is usual to reproduce a master by electricalphotography methods such as, most typically, placing an electrostaticcharge on a photoconductive surface, selectively dissipating such chargeby exposure to an optical image corresponding to the master to bereproduced and developing the resulting electrostatic image by exposureto an electroscopic material. According to one practice in xerography asdisclosed in US. 2,618,552 to E. N. Wise, the development of theelectrostatic image is accomplished by rolling or cascading across theimage-bearing-surfacc a developer composition of relatively largecarrier particles having on their surfaces and electrostatically coatedthereon fine powder particles known as toner particles. As thecomposition cascades or rolls across the image-bearing surface, thesetoner particles are .electrostatically deposited on and secured to thecharged portions of the image and are not deposited on the uncharged orbackground portions of the image. More than that, toner particlesaccidentally deposited on these background portions are physicallyremoved therefrom by electrostatic action of the carrier particlespassing thereacross whereby these toner particles are electrostaticallysecured to the rolling carrier particles and are picked up from thesurface in this manner. The result is an excellent copy of theelectrostatic image in the form of an image .by the toner particleselectrostatically clinging to the image surface and removable therefromby any of various means such as adhesive transfer, electrostatictransfer,-or the like. Thus the image 'body may be transferred to asheet in contactwith the image body and applying an electrostatic chargeto the paper while in such contact. When the paper is subsequentlystripped from the image-bearing surface it carries with it a substantialproportion of the image body to yield a xerographic print whichthereafter may be made permanent by any desired method such as heating,solvent fixing or the like.

After the image is transferred from the image surface to the transfermember, or paper, the image surface may be cleaned and then is ready foruse in a subsequent xerographic cycle. The xerographic plate, afterbeing properly cleaned following a previous xerographic cycle, is in itsoriginal condition and is substantially unimpaired for future use.However, a problem in prior experience has been caused by the fact thatcleaning of the xerographic plate between cycles is unexpectedlydifiicult, due apparently to the strong attraction of the residual tonerparticles to the plate. This is evidenced in two manners; first, by thestubborn adherence of toner particles as such to the plate with theresult that after mechanical cleaning operations substantial amounts ofsuch powder may still remain on the image surface, and, second, by thefact that a film or layer builds up on the plate surface during repeatedcycles and eventually 2,940,934 Patented June 14, 1960 "ice 2 requiresadditional cleaning operations, such as, for example, solvent cleaningor the like.

Either type of residual toner, that is, either toner in its powder form,or toner as a film on the image surface, impairs the subsequentoperation of the xerographi'c plate. Thus, for example, if tonerparticles remain on the plate, they interfere with the subsequent stepsof xerography causing either deletions or background 'dep osition duringsubsequent steps. Perhaps the more serious trouble, however, is thebuilding up of the film which appears to consist of a gradualaccumulation of the toner material on the plate surface, apparently inthe form of a smear as the toner particles are mechanically removedtherefrom. The presence of this film'interferes in many ways. In thefirst place, this film has different electrical properties from those ofthe photoconductive layer on the xerographic plate and thus it tends tointerfere with the charging or sensitizing step. It also has mechanicalor physical properties differing from those of the photoconductivelayer, particularly in that it is substantially more sticky or tackythan the clean plate surface. Beyond this, the toner films appear tobecomewhat hydroscopic to the extent that in humid weather itdetrimentally affects conductivity under exposure to light andinsulating properties in the absence of light, particularly where therelative humidity is greater than about it is presently believed thatagreat :deal of the difficulty causedby the building up ofthis film,and, in fact, by incomplete removal of toner particles from the residualimage is a function of aphysical-or mechanical property of the tonerparticle in that they tend to be somewhat tacky and apparently adhere tothe image surface by mechanical means as well as by electrostaticforces. The'problem, moreover, is complicated by the fact that the usualand presently preferred method of transferring the image body from theimage surface to the transfer member, such as the paper sheet,-preferablyis carried out by electrostatic forces, with the result that:the mechanical adhesion between toner and image surface cannot beovercome simply by increasing the mechanical transfer force. Beyondthis, the 'difiiculties are compounded by the fact that the presentlypreferred fixing method for permanentizing the xerographic print employsheat fusion to melt the toner particle onto and into the surface of thetransfer sheet. Thus, the toner particle must be capable ofelectrostatic transfer and subsequently must be usable withintemperature limits readily tolerated by convenient and economicaltransfer members such as paper or the like, and the usual methods oflowering melting points generally tend to increase tackiness. A furtherandoccasionally nagging difficulty is the fact that the toner particlesmust be charged to correct polarity upon mixing with and coating on thesurface of the carrier particles so that the toner will be deposited onthe image areas by electrostatic attraction andremoved from thenon-image areas also by electrostatic attraction. At the present time,xerographic photosensitive members are generally charged to positivepolarity for sensitization and thus the toner particles must be suchthat they are charged to negative polarity 'by mixing with the carrierparticles. These and other problems must be solved while achieving thenecessary end results of suitability for xerography, including theability to form an ink-receptive image useful in lithography.

Now inraccordance with the present invention there is provided a new andimproved xerographic toner composition comprising finely dividedpigmented resin particles having a particle size less than about 20microns :and preferably having an average particle size between-about 5and about 10 microns and consisting of a finely divided uniform mixtureof pigment in a non-tacky low melting resin consisting substantially ofa polymerized styrene or a blend of polymerized styrenes having amelting point of about 257 F. (ball and ring) and being characterized byextreme toughness as measured by non-smearing properties and by being anextremely hard thermoplastic resin. The polymerized styrene is presentin the composition in a predominating amount which is defined herein asbeing at least about /5 of the entire composition; optionally mixed orblended with up to about 25% of modifying polymeric material either bymechanically mixing the polymers or by combining the monomers andcopolymerizing (which might be considered as a chemical mixture). Thepigment or dye is present in the toner in a sufiicient quantity to causeit to be highly colored whereby it will form a clearly visible image ona transfer member. Thus, for example, in the usual case where axerographic copy of a document or the like is desired, the pigment willbe a black pigment such as carbon black or other minutely dividedcarbonaceous ptgment. Desirably, the pigment is employed in an amount ofat least about 5% based on the total weight of the toner body andgenerally between about 5% and about In the preparation of the tonercomposition according to the present invention the ingredients arethoroughly mixed to form a uniform dispersion of the pigment in the mainbody and thereafter the body is finely divided to form the desired toneror powder composition. The mixing may be done by various means,including combinations of the steps of blending, mixing, and milling andthe presently preferred method includes a step or blending in a rubbermill to assure uniform and fine dispersion of the pigment in the resin.

One particularly important characteristic of the new composition is itseasy cleanability from a surface to which itis clinging by relativelyhigh electrostatic forces. This characteristic is due to a propertygenerally similar to low tackiness and now believed to be attributableto hardness and toughness of the finely divided material. Thus, whenrubbed across a surface or when wiped off a surface, the toughcomposition does not smear or stick to the surface.

One particular type of resin composition which has been found to beunusually well suited to the present invention is a polymerized blendof' styrene and styrene homologues of the general formula where R isselected from the group consisting of hydrogen and lower alkyl. Theresins of this type are prepared in a wide range of average molecularweights from crude mixtures of styrene and styrene homologues such asthose obtained from fractionation of the so-called crude solven fromlight oils scrubbed out of coke oven or gas house gas. The resins arepale in color and are non-acid and unsaponifiable. They aresubstantially wholly hydrocarbon in composition. Polymerization does notadvance significantly with age or heat.

A particularly preferred type of resin within this class is a hard,tough resin having a ring and ball melting point of about 125 C. or 257F. being medium-high in polymer structure. When heated to 150" C. it isquite viscous. A resin of this type, specifically designated in thefollowing examples, is available under the name Piccolastic D-l25. It,and its blends with minor amounts of up to V3 of a blended resin(wherein the predominating amount, or at least is the styrene-type resinthus defined) are hard and tough, highly resistant to ball milling, andmelt below the char point of ordinary paper.

The general nature and scope of the invention having been set forth, thefollowing specific embodiments are presented in illustration but not inlimitation thereof, and it is to be understood that the invention is tobe limited only by the appended claims.

Example L-A mixture was prepared comprising 10% by weight of carbonblack, 25% by weight polybutyl methacrylate, and 65% by weight of ablend of polymerized styrenes. The blend of polymerized styrenes isavailable under the name Piccolastic Resin D-l25 and is believed to be amixture of medium-high molecular weight polystyrenes having a ball andring melting point of 257 F. at room temperature. It is a hard, horny,dry solid, and when heated, can be blended with polybutyl methacrylateto yield a homogeneous mixture.

After melting and preliminary mixing, the composition was fed to rubbermill andthoroughly milled to yield a uniformly dispersed composition ofcarbon black in the resin body. The resulting mixed composition wasthoroughly cooled and then finely subdivided in jet pulverizer to yielda powder composition having an average particle size of about 5 microns.Particle size was moderately uniform with substantially no particleslarger than 10 microns and substantially none less than one micron. Theresulting powder or toner composition is particularly adapted for use inxcrography in combination with vitreous selenium xerographic platescharged to positive polarity for sensitization and exposed to an opticalimage to yield a positive polarity electrostatic image on the seleniumsurfaced xerographic plate.

To illustrate the suitability and effectiveness of the material, thepowder was deposited on an electrostatic latent image on an imagesurface by mixing about 1% of the powder in -a two component developeras described in Patent Number 2,618,551 and cascading the mixture acrossan electrostatic image-bearing surface. The image was developed bydeposition of the powder on the electrostatic image and the powder wastransferred by electrostatic-means to a transfer web such as paperwhereon it was fused byplacing in a heated oven at a temperature of 250F. for a period of 5 seconds. A residual powder image was cleaned offthe image-bearing surface by the method of Patent Number 2,484,782, orby other suitable methods.

The melting point of the resin'powder was determined under theconditions of use in xerography, namely by heating the powdercomposition until it is sufficiently soft to fuse into a single mass andadhere permanently toa web such as a sheet of paper. Its melting pointin this manner was 210 F.- Under these same heating conditions the paperweb is not visibly affected by heat. The residual toner image remainingon the xerographic plate after electrostatic transfer is readily removedby desired methods such as cascading thereacross a cleaning compositionas disclosed in Patent 2,484,782 or by brushing the surface with arapidly rotating fur brush or by like methods. When employed in thexerographie cycle with either of these cleaning operations, the platewas readily cleaned free from detectable residual toner particles andcould be recycled for at least 1,000 xerographic cycles without buildingup significant quantities of film on the surface of the xerographicplate.

When mixed with a carrier composition surfaced with a suitable resinsuch as is disclosed in Patent Number 2,618,551 the toner preparedaccording .to this example gave clear sharp and extremely black imagesof satisfactory contrast resolution and appearance. The toner particleswhen mixed with this carrier composition acquired a negative polarityelectrostatic charge whereby they were deposited on a positive polarityelectrostatic image and removed by the carrier from uncharged surfaceareas.

Example 2.--The procedure of Example 1 was repeated employing a mixtureof of the same blend of polymerized styrenes and 10% carbon black. Inthis case the final composition was finely subdivided to yield a powdercomposition having an average particle size of about microns withsubstantially none greater than 20 microns. The product was closelycomparable in behavior and properties with the product of Example 1,characterized, however, with a slightly lower melting point, namelyabout 190 F., whereby it melted at a temperature having a greater marginof safety below the char point of paper.

Example 3.The procedure of Example 1 was repeated employing as themixture 10% carbon black, 5% polybutyl methacrylate and 85% of theblended polystyrenes. The product had a melting point of about 250 F.,and, like the product of Example 1, could be used through at least 1,000xerographic cycles without building up a coating of film on thexerographic plate. Example 4.The procedure of Example 1 was repeatedemploying as the mixture 10% carbon black, 5% of a polyamide resin soldby E. I. du Pont de Nemours and Company under the trade name Nylon FM6501," and 85% of the blended polystyrenes. The product had a meltingpoint of about 250 F. and like the product of Example 1, could be usedthrough many hundred xerographic cycles without building up anobjectionable coating of film on the xerographic plate.

Example 5.-The procedure of Example 1 was repeated employing as themixture 10% carbon black, 5% of a polyethylene resin sold by UnionCarbide and Carbon Corp., under the trade name Vinylite DYLT, and 85% ofthe blended polystyrenes. The product had a melting point of about 250F. and like the product of Example 1 could be used through many hundredxerographic cycles without building up an objectionable coating of filmon the xerographic plate.

Example 6.--The procedure of Example 1 was repeated employing as themixture 10% carbon black, 10% of a polyvinyl butyral sold by the UnionCarbide and Carbon Corp. under the trade name Vinylite XYHL," and 80% ofthe blended polystyrenes. The product had a fusing point of about 245F., and like the product of Example 1 could be used through at least1000 xerographic cycles without building up an objectionable coating offilm on the xerographic plate.

Example 7.The procedure of Example 1 was repeated employing as themixture 10% carbon black, 10% of a copolymer of butadiene and styrenesold under the name GR-S 1500, and 80% of the blended polystyrenes. Theproduct had a fusing point of about 260 F., and like the product ofExample 1 could be used through at least 1000 xerographic cycles withoutbuilding up an objectionable coating of film on the xerographic plate.

Example 8.-The procedure of Example 1 was repeated employing as themixture 10% carbon black, 25% polystyrene and 75% of the blendedpolystyrenes. The product had a fusing point of about 270 F. and likethe product of Example 1 could be used through at least 1000 xerographiccycles without building up an objectionable coating of film on thexerographic plate.

Example 9.-The procedure of Example 1 was repeated employing as themixture 10% carbon black, 25% of a polyvinyl acetal obtained from theShawinigan Products Corporation under the trade name Alvar 7-70, and 65%of the blended polystyrenes. The product had a fusing point of about 270F., and like the product of Example 1, could be used through at least1000 xerographic cycles without building up an objectionable coating offilm on the xerographic plate.

Example 10.The procedure of Example 1 was repeated employing as themixture 10% carbon black, 25 of a copolymer of vinyl chloride and vinylacetate obtained from the Union Carbide and Carbon Corp. under the tradename Vinylite VYHH-l, and 65% of the blended polystyrenes. The producthad a fusing point of about 225 F., and like the product of Example 1 6could be used through at least 1000 xerographic cycles without buildingup an objectionable coating of film on the xerographic plate.

Example 11.--The procedure of-Example l was repeated employing as themixture 80% of a hydrocarbon resin obtained from the VelsicolCorporation under the trade name Velsicol AD-6-3, and 20% polystyrene.The toner so prepared was mixed with xerographic carrier particles usedto develop electrostatic images on a xerographic selenium plate asdescribed in Example 1. The resulting powder image was transferred to apaper support sheet using electrostatic transfer as described in U.S.2,576,047 to R. M. Schaffert, and fixed thereto by heating. The residualtoner remaining on the selenium surface was removed by rubbing thesurface with a cotton swab. After 20 such cycles a film had built up onthe surface of the selenium plate of such thickness and density as tocompletely obscure the surface of the plate. No image whatsoever couldbe developed on this plate after the 20 cycles due to the heavy film.The process was then repeated using as the toner the composition ofExample 1. After some 500 cycles a slight film had built up on theselenium surface of the xerographic plate. The selenium was stillperfectly visible and the plate was still producing excellent qualityimages. Accordingly, the toner of Example 1 was further tested on anautomatic machine using a cylindrical drum coated with selenium as thexerographic plate. The drum was cleaned by means of rubbing with arapidly rotating rabbits fur brush. The apparatus was run continuouslyfor 24 hours. At the end of that time, although the machine was stillproducing perfectly legible copy, the machine was stopped and theselenium surface inspected. A detectable film was noticeable on the drumbut despite the heavy rubbing the selenium surface was perfectlyvisible. In this 24-hour period, the drum had gone through over 7000revolutions.

The novel toner compositions of the instant invention have exceptionalutility in developing electrostatic images. In the normal xerographicprocess such images are created and developed on the surface of aphotoconductive insulating member which is generally amorphous selenium.Other photoconductive insulating materials include photoconductivepigments as zinc oxide, zinc-cadmium sulfide, tetragonal lead monoxide,titanium dioxide, etc., in an insulating resin binder. Such materialsmay also be used as the photoconductive insulating layer.

In addition, rather than developing the electrostatic image on thephotoconductive insulating layer, if desired, the electrostatic imagepattern may be transferred to an electrically insulating film aspolyethylene terephthalate. This process is described more fully in U.S.patent application Ser. No. 434,491, filed on June 4, 1954, now U.S.Patent No. 2,825,814, by L. E. Walkup. The toner composition of theinstant invention may then be used to develop the electrostatic image onthis insulating film to form a powder image corresponding to theelectrostatic image thereon. The resulting powder image may then beeither permanently afiixed to the insulating film or transferred to asupport member as paper, metal, plastic, etc., and the insulating filmcleaned and reused in the process.

As used in developing electrostatic images the toner composition isloosely coated on a carrier surface to which it remains loosely aflixedby reason of electrostatic attraction thereto. The most widely usedmethod of carrier development is known as cascade carrier development asmore fully described in U.S. 2,618,551, to L. E. Walkup; U.S. 2,618,552,to E. N. Wise; and U.S. 2,638,416, to Walkup and Wise. In this processthe electroscopic toner is desirably mixed with a granular carrier,either electrically conducting or insulating, magnetic or non-magnetic,provided that the particles of granular material when brought in closecontact with the powder particles acquire a charge having an oppositepolarity to that of the powder particles adhere to and surround thegranular carrier particles. If a positive reproduction of theelectrostatic image is desired, the carrier is selected so that thetoner particles acquire a charge having the opposite polarity to that ofthe electrostatic image. Alternatively, if a reversal reproduction ofthe electrostatic image is desired the carrier is selected so that thetoner particles acquire a. charge having the same polarity as that ofthe electrostatic image. Thus, the materials for the granular materialare selected in accordance with their tr-iboelectric properties inrespect to the electroscopic toner so that when mixed or brought intomutual contact one material is charged positively if the other is belowit in a tribeelectric series, and negatively if the other material isabove it in a triboelectric series. By selecting materials in accordancewith their triboelectric efiects, the polarities of their charge whenmixed are such that the electro scopic toner particles adhere to and arecoated on the granular carrier particles and also adhere to theelectrostatic image on the plate which thus retains the electroscopictoner in the charge areas that have a greater attraction for the tonerthan the granular carrier particles have.

The granular carrier particles are grossly larger than the tonerparticles by at least one order of magnitude of size, and are shaped toroll across the image-bearing surface. Generally speaking, the carrierparticles should be of suflicient size so that their gravitation ormomentum force is greater than the force of attraction of the toner inthe charged areas where'the toner is retained on the plate in order thatthe granular carrier particles will not be retained by the tonerparticles, while, at the same time, the toner particles are attractedand held, or repelled, as the case may be, by the charge or unchargedareas of the plate since they acquire a charge of opposite polarity tothe charge of both the granular carrier particles and the plate. It hasbeen found best to use granular carrier particles of a size larger thanabout 200 mesh, usually between about 20 and about 100 mesh, and tonerparticles of a size from about 1 to 20 microns. The granular carrierparticles may, if desired, be somewhat larger or smaller as long as theproper size relationship to the electroscopic toner is maintained sothat the granular carrier particles will flow easily over the imagesurface by gravity when the plate is inclined without requiringadditional means or measures to remove them.

The degree of contrast or other photographic qualities in the finishedimage may be varied by changing the ratio of granular carrier toelectroscopic material. Successful results have been had with from aboutto about 200 parts by weight of granular carrier particles capable ofbeing passed through a 30 mesh screen and being collected on a 60 meshscreen to 1 part of the electroscopic toner having a particle size of 1to 20 microns. Generally speaking, carrier-to-toner ratios in the orderof about 100 to 1 prove satisfactory and preferred compositions run fromabout 70 to 1 to about 150 to 1. In such preferred compositions thecarrier acts efiectively to remove any toner particles which might tendto adhere to a non-image area and the toner itself forms a dense readilytransferable and fusible image.

In addition to the use of granular particles to provide the carriersurface, the bristles of a fur brush may also be used. Here also, thetoner particles acquire an electrostatic charge of polarity determinedby the relative position of the toner particles and the fur fibers inthe tn'boelectric series. The toner particles form a coating on thebristles of the fur clinging thereto by reason of the electrostaticattraction between the toner and the fur just as the toner clings to thesurface of the granular carrier particles. The general process of furbrush development is described in greater detail in U.S. patentapplication, Ser. No. 401,811, filed by L. E. Walkup on January 4 1954.

Even more closely related to the cascade carrier development is magneticbrush development. In this process a granular carrier is selected havingferromagnetic properties and selected relative to the toner in atribeelectric series so as to impart the desired electrostatic polarityto the toner and carrier as in cascade carrier development. On insertinga magnet into such a mixture 7 of toner and magnetic granular materialthe carrier particles align themselves along the lines of force of themagnet to assume a brush-like array. The toner particles areelectrostatically coated on the surface of the granular powder carrierparticles. Development proceeds as in regular cascade carrierdevelopment on moving the mag net over the surface bearing theelectrostatic image so that the bristles of the magnetic brush contactthe electrostatic image-bearing surface.

Still another method of carrier development is known as sheet carrierdevelopment in which the toner particles are placed on a sheet orpellicle as of paper, plastic, or metal. This process is described inU.S. patent applica-. tion' Ser. No. 399,293, filed by C. R. Mayo onDecember 21, 1953. As described therein the electrostatic attractionbetween the sheet surface and toner particles necessary to assureelectrostatic attraction therebetween may be obtained by leading thesheet through a mass of the electroscopic toner particles whereby thereis obtained a rubbing or sliding contact between the sheet and thetoner. In general it is desirable to spray the surface of the sheetbearing the electroscopic toner particles with ions of the desiredpolarity as by the use of a corona charging device as described in theapplication of Mayo. In any event,

as used in developing an electrostatic image the toner composition oftheinstant invention is electrostatically coated on a suitable carriersurface which is then in turn contacted with the surface bearing theelectrostatic image whereby the electroscopic toner particles aretransferred to the surface bearing the electrostatic image to formthereon a powder image corresponding to the electrostatic image.

This application is a continuation-in-part of my copending applicationSer. No. 363,381, filed June 22, 1953, now abandoned.

I claim:

1. An improved xerographic developer comprising a mixture offinely-divided xerographic toner particles elw trostatically coated oncarrier particles, the carrier particles being shaped to roll across asurface and at least about 200 mesh and having removably coated thereonby electrostatic attraction toner powder particles having a particlesize on the average less than about 20 microns, said toner powderparticles consisting essentially of from about 5 to about 10 percentpigment and 95 to percent resin containing'at least two-thirdspolystyrene based on the total weight of the toner, the finely-dividedtoner being hard and tough and being thereby adapted to be easilycleaned by brushing from an insulating surface.

2. An improved xerographic developer comprising a mixture offinely-divided xerographic toner particles electrostatically coated oncarrier particles, the carrier particles being shaped to roll across asurface and at least about 200 mesh and having removably coated thereonby electrostatic attraction toner powder particles having a particlesize on the average less than about 20 microns, said toner powderparticles consisting essentially of from about 5 to about 10 percentpigment and to 90 percent resin containing at least two thirds based onthe total weight of the toner of a polymerized blend of monomers ofstyrene and styrene homologs of the formula where R; is selected fromthe group. consisting of hydrogen and-lower alkyl, the polymerizedblend. of styrene monomershaving a.ring and ball melting point of about125 C., the toner particles being hard and tough and being therebyadapted to be easily cleaned, by brushing, from an insulating surface.

3. An improved xerographic developer comprising a mixture offinely-divided xerographic toner particles electrostatically coated oncarrier particles, the carrier particles being shaped to roll across asurface and at least about 200 mesh and having removably coated thereonby electrostatic attraction toner powder particles having a particlesize on the average less than about 20 microns, said toner powderparticles consisting essentially of from about to about percent pigmentin a polystyrene resin having a ring and ball melting point of about 125C.

4. An improved xerographic developer comprising a mixture offinely-divided xerographic toner particles electrostatically coated oncarrier particles, the carrier particles being shaped to roll across asurface and at least about 200 mesh and having removably coated thereonby electrostatic attraction toner powder particles having a particlesize on the average less than about microns, said toner powder particlesconsisting essentially of from about 5 to about 10 percent pigment in apolymerized blend of monomers of styrene and styrene homologues of theformula R where R is selected from the group consisting of hydrogen andlower alkyl, the polymerized blend of styrene mono mers having a ringand ball melting point of about 125 C., the toner particles being hardand tough and being thereby adapted to be easily cleaned, by brushing,from an insulating surface.

5. An improved xerographic developer comprising finely-divided powderparticles uniformly electrostatically coated on a carrier surfacecapable of retaining said powder particles by electrostatic attraction,the carrier surface being adapted to make firm contact with a surfacebearing an electrostatic image and having removably coated thereon byelectrostatic attraction xerographic toner particles having a particlesize on the average less than about 20 microns, said toner powderparticles consisting essentially of from about 5 to about 10 percentpigment and 95 to 90 percent resin containing at least twothirdspolystyrene based on the total weight of the toner, a finely-dividedtoner being hard and tough and being thereby adapted to be easilycleaned, by brushing, from an insulating surface.

6. An improved xerographic developer comprising finely-divided powderparticles uniformly electrostatically coated on a carrier surfacecapable of retaining said powder particles by electrostatic attraction,the carrier surface being adapted to make firm contact with a surfacebearing an electrostatic image and having removably coated thereon byelectrostatic attraction xerographic toner particles having a particlesize on the average less than about 20 microns, said toner powderparticles consisting essentially of from about 5 to about 10 percentpigment and 95 to 90 percent resin containing at least twothirds basedon the total weight of the toner of a polymerized blend of monomers ofstyrene and styrene homologues of the formula R 10 where R isselectedfrom the group consistingot hydrogen and lower alkyl, the. polymerizedblend of styrene. monomers having ariug and ballmelting point of about125 C., the toner particles being hard and tough and being therebyadapted to be easily cleaned, by' brushing, from an insulating surface.

7. An improved xerographic developer comprising finely-divided powderparticles uniformly electrostatically coated on a carrier surfacecapable of retaining said powder particles by electrostatic attraction,the carrier surface being adapted to make firm contact with a surfacehearing an electrostatic image and having removably coated thereon byelectrostatic attraction xerographic toner particles having a particlesize on the average less than about 20 microns, said toner powderparticles consisting essentially of from about 5 to about 10 percentpigment in a polystyrene resin having a ring and ball melting point ofabout 125 C.

8. An improved xerographic developer comprising finely-divided powderparticles uniformly electrostatically coated on a carrier surfacecapable of retaining said powder particles by electrostatic attraction,the carrier surface being adapted to make firm contact with a surfacebearing an electrostatic image and having removably coated thereon byelectrostatic attraction xerographic toner particles having a particlesize on the average less than about 20 microns, said toner powderparticles consisting essentially of from about 5 to about 10 percentpigment in a polymerized blend of monomers of styrene and styrenehomologs of the formula where R is selected from the group consisting ofhydrogen and lower alkyl, the polymerized blend of styrene monomershaving a ring and ball melting point of about 125 C., the tonerparticles being hard and tough and being thereby adapted to be easilycleaned, by brushing, from an insulating surface.

9. A process of xerography wherein an electrostatic image is madevisible comprising contacting an image surface bearing an electrostaticimage thereon with a finelydivided pigmented resin consistingessentially of between about 5 and about 10 percent pigment and 95 topercent resin containing at least two-thirds polystyrene based on thetotal weight of the toner, a finely-divided toner being hard and toughand being thereby adapted to be easily cleaned, by brushing, from aninsulating surface.

10. A process of xerography wherein an electrostatic image is madevisible comprising contacting an image surface bearing an electrostaticimage thereon with a finelydivided pigmented resin consistingessentially of between about 5 and about 10 percent pigment and to 90percent resin containing at least two-thirds based on the total weightof the toner of a polymerized blend of monomers of styrene and styrenehomologs of the formula where R isselected from the group consisting ofhydrogen and lower alkyl, the polymerized blend of styrene monomershaving a ring and ball melting point of about C., the toner particlesbeing hard and tough and being thereby adapted to be easily cleaned, bybrushing, from an insulating surface.

11. A process of xerography wherein an electrostatic image is. madevisible comprising contacting an image surface bearing an electrostaticimage thereon .with a finelydivided pigmented resin'consistingessentially of between about 5 and about 10 percent pigment in apolystyrene resin having a ring and ball melting point of about 12. Aprocess of xerography wherein an electrostatic image is made visiblecomprising contacting an imagesurface bearing an electrostatic imagethereon with a finely-- where R is selected from the group consisting ofhydrogen and lower alkyl, the polymerized blend of styrenemonomers'having a ring and ball 'melting point of about 125 C., thetoner particles being hard and tough and being thereby adapted to beeasily cleaned, by brushing, from an insulating surface.

References Cited in the file of this patent UNITED STATES PATENTSRheinfrank et al Apr. 9, 1957'

2. AN IMPROVED XEROGRAPHIC DEVELOPER COMPRISING A MIXTURE OFFINELY-DIVIDED XEROGRAPHIC TONER PARTICLES ELECTROSTATICALLY COATED ONCARRIER PARTICLES, THE CARRIER PARTICLES BEING SHAPED TO ROLL ACROSS ASURFACE AND AT LEAST ABOUT 200 MESH AND HAVING REMOVABLY COATED THEREONBY ELECTROSTATIC ATTRACTION TONER POWDER PARTICLES HAVING A PARTICLESIZE ON THE AVERAGE LESS THAN ABOUT 20 MICRONS, SAID TONE POWDERPARTICLES CONSISTING ESSENTIALLY OF FROM ABOUT 5 TO ABOUT 10 PERCENTPIGMENT AND 95 TO 90 PERCENT RESIN CONTAINING AT LEAST TWO-THIRDS BASEDON THE TOTAL WEIGHT OF THE TONER OF A POLYMERIZED BLEND OF MONOMERS OFSTYRENE AND STYRENE HOMOLOGS OF THE FORMULA